The invention relates to a method for the analytic determination of physical characteristics of a material. More specifically, the invention relates to determining the "granulometry" of a flowing or falling stream of material such as granular material or a dispersed liquid moving in the form of droplets. This method is especially suitable for determining the grain size of dry solid materials such as aggregate or for determining the size of falling drops of liquid. The material in the case of solid materials may be stationary in a measuring zone or the material such as granular material or liquid droplet may move between a delivery station and a receiving station. The physical characteristics, once they have been determined may be used for further evaluation or display and for control purposes, for example, in connection with the control of the material supply through the measuring zone.
The term "granulometry" in this context is supposed to mean the ascertaining of the various size ranges of particles making up a granular material such as aggregate.
The German Patent (DT-PS) No. 1,140,355 describes a weight monitoring apparatus for maintaining the mixing ratio of a dry component in a mixture. In said prior art method a predetermined partial quantity of material is intermittently removed from a flowing stream of material and sorted according to size. The result of such sorting is used to control, in a closed loop fashion, the output or discharge of the material from the individual supply bins.
In this known apparatus, a collection container is arranged downstream of each sorting device for each material component. The outflow of all collection containers is supplied into a common weighing apparatus. The content of the collection containers is weighed one after the other, and the result of the weighing is used as a control value for controlling the outflow of the supply bins.
Such prior art method has a number of substantial disadvantages. For example, the time for the sorting, weighing and evaluating for the purpose of correcting the supply of material is relatively long. Hence, the output of the supply bins in response to the sorting result may occur only after several minutes have passed, at which time a stream of material is being controlled, which already deviates considerably from the sorting result based on the prior sample taking.
The use of said prior art apparatus in connection with a preparation plant for the manufacture of a bituminous mixture is especially critical. Since only dried mineral materials may be used for making a reasonably exact screen analysis, the sample may be taken only after passage of the mineral materials through a rotary drum dryer, the resulting correction time is often in the order of about ten minutes. In other words, a condition is being changed or influenced which had already appeared ten minutes earlier, and generally is no longer present. Hence, the known apparatus is not useful for the purpose of screen analysis.
Furthermore, the true control of components ranging in size from 0 to 2 mm is not possible. Take, for example, the mean values of the constituents ranging from 0 to 2 mm for a so-called asphalt fine concrete which is rich in crushed stone according to governmental regulations in the Federal Republic of Germany. Accordingly, after the removal of the filler material the mean grading curve of the sand component is made up as follows.
TABLE I ______________________________________ sand component size in 0-2 asphalt fine concrete Constitutent size (mm) min max % ______________________________________ 0.09 0.25 12 0.25 0.71 13.5 0.71 2.00 13.5 Total sand in the mixture 39% ______________________________________
In the case of asphalt fine concrete which is poor in crushed rock and which has a sand component in the range of 0 to 8 mm, the following picture may be obtained for the sand component again after removal of the filler: results:
TABLE II ______________________________________ sand component size in 0 to 8 mm asphalt fine concrete component range in mm min max % ______________________________________ 0.09 0.25 25.5 0.25 0.71 18.5 0.71 2.00 17.0 Total sand in the mixture 61% ______________________________________
The foregoing tables mean that the composition of 40 to 60% of the mixture portion is not controllable by means of a quick screen analysis. In the case of mixtures rich in crushed stone, the problem is somewhat simpler since in such mixtures the sand is made up of three approximately equal portions. The problem is more critical in the case of asphalt mixtures poor in crushed stone, since the fine sands in the size range from 0.09 to 0.25 mm are present in a considerably larger proportion in comparision to the two other components of the total sand proportion.
If one considers all of the factors influencing the degree of quality of the screen analysis, such as the method of operation of the screening machine, the type of granularity, the mesh size, the screening duration, and particularly the quantity of screened material or the screen throughput, the monitoring of a stream of granular solid materials to be sorted with respect to its required granulometry becomes even more problematic particularly in the case of fluctuating throughputs. Although the above examples relate to bituminous mixtures the respective conclusions are relevant to the preparation or the bringing together of all building material mixtures. Beyond that, these conclusions are relevant to the combination of all solids in various classes of granulation or grain sizes.